1. Field of the Invention
This invention is an improved technique for producing optical fibers. The technique eliminates a number of steps heretofore required in the manufacture of preforms from which the fibers are to be drawn. In addition, the technique allows for the formation of fibers with highly resolved longitudinal gradations in index of refraction. Such gradations are known to enhance mode conversion, thereby reducing mode dispersion.
2. Description of the Prior Art
The many advantages of optical communications, both potential and realized, have stimulated significant efforts directed towards further development of this field of technology. The advantages of optical communications have long been apparent from basic theoretical considerations. However, it was not until the discovery of the laser that the development of this technology began in earnest. This may be attributed to the improved efficiency of optical communications when combined with coherent light sources.
Although the realization of a totally optical communication system still seems far in the future, the advantages of optical transmission alone are sufficiently impressive to warrant considerable effort in the development of optical transmission systems. For example, the high frequency associated with visible radiation allows for a significant increase in bandwidth over that available in normal electrical transmission systems. However, to transmit light and utilize this increased bandwidth, a medium capable of supporting optical signals must be developed. Basic electromagnetic theory indicates that light traversing a medium of index of refraction n.sub.1 will not be transmitted through an interface with a medium of index of refraction n.sub.2, if n.sub.1 is greater than n.sub.2, and if the angle that the light ray makes with the interface is less than arc cosine n.sub.1 /n.sub.2. Under such circumstances the light is contained in the medium n.sub.1 and will be transmitted through this medium. This basic principle has led to the development of glass fibers for use as optical transmission lines. In this development a significant hurdle to be overcome has been the development of fibers whose optical loss is low enough for practical applications. Losses smaller than 50 db/km are necessary for short length transmission, while losses of a few db per km or less are necessary for long distance transmission.
At the present time glass deposition fiber fabrication techniques are found to be the most economically feasible while at the same time yielding the requisite low loss qualities. Two different deposition processes are now being considered for widespread use--the "soot" deposition technique, and the modified chemical vapor deposition technique. In the soot deposition technique a gas vapor mixture is hydrolyzed within a flame to form a glass precursor particulate, or soot. The gas vapor is produced, for example, by bubbling oxygen through chlorides, hydrides or other compounds of silica, or any desired dopant such as germanium. The glass soot, formed by the hydrolysis of the vapor in the burner, is deposited on a rotating glass rod which serves as a mandrel. The soot is deposited from a direction perpendicular to the mandrel and the burner is translated parallel to the mandrel axis during the deposition. In this manner successive layers of constant radius are formed. A preform with radial gradations in index of refraction, for improved transmission characteristics, may be produced in like manner by changing the dopant concentration on each successive pass of the burner. When a sufficient amount of soot has been deposited the preform is heated in an oven in order to consolidate the soot into a unitary glass body. The rod mandrel is then removed and the cylindrical preform is collapsed to a solid rod and drawn into an optical fiber using flame, oven, or laser techniques well known in the art. This process for producing optical preforms is discussed in detail in U.S. Pat. Nos. 3,826,560 and 3,823,195.
In certain applications it is desirable to use optical fibers with longitudinal gradations in index of refraction. As will be discussed later such fibers display improved "mode conversion" transmission characteristics. In the "soot" technique, as it is commonly practiced, the longitudinal resolution of any material gradations is limited by the breadth of the flame and the particulate stream. The breadth of the flame is of the order of centimeters and this technique is, consequently, not capable of forming the micron gradations necessary in longitudinally graded fiber preforms.
The second prevalent technique for producing optical fibers is the modified chemical vapor deposition technique (MCVD). This technique is described in a commonly assigned application Ser. No. 444,705. In this technique a gas vapor mixture is directed through a glass tube. The gas vapor includes the normal glass precursor vapors as described above. A ring of heaters surrounds the outside of the tube and traverses it from one end to the other while the tube is rotated in a glass lathe. As the vapor passes that section of the cylinder which is being heated, the vapor forms particulate matter which drifts downstream, settles on the tube's inner wall, and is subsequently fused onto the inner surface of the glass tube, thereby forming a unitary glass structure. The burner traverses the rod at a predetermined rate and makes numerous successive passes depending on the amount of deposition required. After a sufficient deposit has built up the tube is collapsed to a solid preform and subsequently drawn into a fiber.
In the MCVD process gradations in index of refraction can be produced by varying the dopant concentration in the gas mixture flowing down the center of the cylinder. In such a manner, for example, radial gradations in index of refraction may be produced by changing the dopant concentration for each successive pass of the burner ring. It is clear, however, that in this technique any attempt at producing longitudinal gradations in index of refraction is limited by the diffusion characteristics of the particulate matter that is formed within the cylinder. This restriction places an upper limit on the amount of longitudinal resolution, that is, the size of the gradation, which may be produced.
Since a specific embodiment of the instant invention involves the fabrication of longitudinally graded fibers, an understanding of the role of such fibers in optical transmission will aid in the appreciation of the invention. The need for fibers with gradations in index of refraction especially arises when, in order to more effectively carry information, the envisioned optical signal is in the form of optical pulses. Such pulses must be individually resolvable at the detecting end of the transmission line, as they were at the launching end. A number of phenomena, however, tend to broaden the pulses and consequently degrade the resolution. One of these phenomena is the frequency dispersion effect. As a result of this effect, light of different frequencies travels at different speeds within the fiber. Consequently the different frequency components in an optical pulse of light travel at different velocities -- arriving at the detector at different times, thereby broadening the pulse. The use of highly monochromatic light, for example from a laser, helps to alleviate the frequency dispersion problem.
However, in addition to frequency dispersion, there is a serious mode dispersion effect. This effect may be understood by considering the different paths that a given light ray may take as it traverses the optical fiber. It may, for example, proceed directly down the center of the fiber. On the other hand, it may reflect off the fiber walls numerous times as it traverses the fiber. Each of these possible paths, referred to as modes, has a different path length and consequently the traversal time associated with each of these modes is different. Different pulses will traverse the fiber in different times depending upon the mode in which they are transmitted. In addition, different components of a given pulse will traverse the fiber in different modes and hence in different times. These effects result in a general broadening of the pulses, and a consequent loss of pulse resolution and hence bandwidth. Such effects are referred to by the term mode dispersion.
Initial attempts to alleviate the mode dispersion problem involved the fabrication of single mode fibers. Such fibers will support only one specific mode, thereby eliminating mode dispersion. Technical difficulties were, however, encountered with single mode fibers. Launching an optical signal into a small diameter signal mode fiber entails severe restraints on the coupling system between the source and the fiber. In addition, single mode fibers cannot efficiently transmit light produced by incoherent sources such as the common light-emitting diodes. Since such light sources are simpler and more economical than lasers, considerable interest has centered about multimode waveguides which can efficiently transmit such light. However since such a fiber can support many different modes the mode dispersion effect is an important consideration, and advantageously is minimized in order to maximize the information carrying capacity of the fiber.
The earliest reductions in mode dispersion in multimode fibers were effected by means of radial gradations in the index of refraction of the fibers. Under such conditions the velocity of light may be greater near the surface of the fiber than at the center. Hence the longer path length modes, which are concentrated at the fiber surface, have greater velocities than the shorter path length modes which are concentrated at the fiber core. Under such conditions the difference in velocity from mode to mode compensates for the different path lengths in the various modes and results in a single traversal time applicable to all of the modes. Fabrication of fibers designed to yield this effect are discussed in U.S. Pat. Nos. 3,823,995 and 3,826,560.
In an article by S. D. Personic in the Bell System Technical Journal, Volume 50, No. 3, March 1971, page 843, an alternative technique for alleviating mode dispersion effects is suggested. Personic shows that while the pulse broadening associated with mode dispersion increases proportionately with the length of the fiber, efficient intentional mode conversion results in a broadening effect which is proportional only to the square root of the fiber length. Stimulated by this finding, numerous studies were made to determine the most effective techniques for enhancing mode conversion. One particular method involves the introduction of gradations in the index of refraction of the fiber along the longitudinal direction. It has been found, however, that to maximize the mode conversion while maintaining radiation loss mechanisms within tolerable limits the spatial periods of such gradations must be between 1 and 10 millimeters. To achieve such spatial gradations while using otherwise conventional fiber drawing techniques, the preform from which the fiber is pulled must have spatial gradations of the order of microns. No practical fabrication techniques have been available to produce gradations of such high resolution. The instant application may be utilized in the fabrication of optical fibers with such highly resolved longitudinal gradations in index of refraction.